Why call it Miller?
The Unix toolkit was created in the 1970s and is a mainstay to this day. Miller's look and feel adheres closely to the classic toolkit style: if this were music, Miller would be a tribute album. Likewise, since commands are subcommands of the
mlr executable, the result is a band, if you will, of command-line tools. Put these together and the namesake is another classic product of the 1970s: the Steve Miller Band.
(Additionally, and far more prosaically ... just as a miller is someone who grinds and mixes grain into flour to extend its usefulness, Miller grinds and mixes data for you.)